The present invention relates to a holding apparatus for holding an optical element, and, more particularly, to an optical element holding apparatus for a projection optical system of an exposure apparatus which is used in a photolithography step performed during a process for fabricating micro devices, such as a semiconductor device, a liquid crystal display device, an imaging device, or an thin-film magnetic head, or in a process for forming masks, such as reticles and photomasks.
As shown in FIGS. 1 and 2, a conventional optical element holding apparatus 200 has a ring-like frame 202, which houses an optical element 201, such as a lens. Three bearing surfaces 204, which support the optical element 201, are arranged on the inner wall of the frame 202 at equiangular distances. Screw holes 205 are formed in the top surface of the frame 202 at positions corresponding to the bearing surfaces 204. Bolts 207 are fastened into the three screw holes 205 by means of three respective clamp members 206.
The fastening of the bolts 207 clamps a peripheral flange 201a of the optical element 201 between the clamp members 206 and the bearing surfaces 204. This holds the optical element 201 in the frame 202 at a predetermined position. Even when force is applied to the optical element 201 in a direction intersecting the optical axis of the optical element 201, the clamp members 206 prevent the optical element 201 from being displaced and stably hold the optical element 201.
Due to the miniaturization of semiconductor device patterns, a projection optical system of, for example, an exposure apparatus used to fabricate semiconductor devices is required to have higher resolution. To acquire higher resolution, it is important that the optical element 201 be fixed when maintaining the optical performance of the optical element 201 of the projection optical system. That is, to maintain the optical performance of the optical element 201, changes in the surface accuracy of the optical surface of the optical element 201 must be minimized when the optical element 201 is supported in the lens barrel of the exposure apparatus.
The optical element 201 is placed on the three bearing surfaces 204 and is clamped by the clamp members 206. The surface shape of the clamped optical element 201 is affected significantly by the machining accuracy of the bearing surfaces 204 (the positional relationship of the bearing surfaces 204), the clamp members 206, and the peripheral flange 201a of the optical element 201. In other words, machining must be performed with extremely high accuracy to make the bearing surfaces 204 identical and arrange the bearing surfaces 204 in the vicinity of the ideal positions so that such affect becomes subtle enough to be negligible. In addition to the bearing surfaces 204, the clamp members 206 and the peripheral flange 201a must also be machined with extremely high accuracy. This makes the machining of the individual members very troublesome and increases manufacturing costs.
Even if the bearing surfaces 204, the clamp members 206, and the peripheral flange 201a were machined with extremely high accuracy, the frame 202 may slightly be distorted when the frame 202 clamping the optical element 201 is fitted in the lens barrel. Such distortion slightly changes the positional relationship of the bearing surfaces 204 and distorts the optical surface of the optical element 201. This lowers the optical performance of the optical element 201.
Further, the pattern miniaturization requires a projection optical system, which has extremely low wave front aberration and distortion. To meet such requirement, the optical axis of the optical element 201 must be aligned as accurately as possible in the projection optical system.
To accurately position the optical element 201 and align the optical axis of the optical element 201, an outer surface 202a and a bottom surface 202b of the frame 202 are connected with an inner wall and receiving portion of the lens barrel. Thus, there is subtle freedom when the frame 202 is attached to the lens barrel. It is therefore necessary to attach the frame 202 to the lens barrel with meticulous care. This is troublesome.
Furthermore, if the frame 202 is inserted in the lens barrel in a slightly tilted state and fitted to the lens barrel with excessive load applied to the frame 202, the frame 202 may be distorted. The distortion of the frame 202 produces an unpredictable stress on the optical element 201, thus lowering the accuracy of the optical surface of the optical element 201.